With the recent miniaturization and densification of electronic equipment, the pattern widths and spacing of printed wiring boards have become smaller. As a result, the copper foil used has tended to become thinner, for example, 12 .mu.m thick foils, rather than conventional 35 .mu.m and 18 .mu.m foils. Recently, the need for this copper foil has increased and ultra-thin copper foils have been tried. Handling a copper foil of 12 .mu.m or less, however, is difficult. For example, it can wrinkle or tear while being produced and/or handled. Similar problems occur when an ultra-thin copper foil is used as the outer layer of a multi-layer printed wiring board. A method of preventing these problems with handling ultra-thin copper foil is needed.
There has been previously proposed a composite foil in which an ultra-thin copper foils is supported on a metal carrier layer so as to allow the copper foil to be separated from the carrier layer. Several carrier metals and types of release layers have been suggested. Producing printed wiring boards from such supported ultra-thin copper foil(s) could be done by electrolytic deposition of a copper layer having a thickness of 1-12 .mu.m onto a metal carrier layer having a thickness of 18-70 .mu.m. then applying the surface of the copper layer to a prepreg, such as a glass-reinforced epoxy resin or the like, and laminating by hot pressing. Finally, the metal carrier layer would be separated, leaving a copper-clad laminate from which a printed wiring board can be made.
When a copper sheet is used as a carrier, a chromium layer may be used as a release layer between the copper foil and the copper carrier layer, as has been disclosed in, for example, Japanese Patent Application Publication (Examined) No. Sho 53-18329.
Alternatively, when aluminum is used as the carrier layer, several types of release layers have been proposed, for example:
1. a release layer of the sulfides or oxides of Cr, Pb, Ni and Ag (for example, in U.S. Pat. No. 3,998,601); PA1 2. a release layer formed of nickel or nickel alloy plating after zinc immersion (for example, in U.S. Pat. No. 3,936,548); PA1 3. a release layer of aluminum oxide (for example, in Japanese Patent Application Publication (Examined) No. Sho 60-31915 and U.K. Patent No. GB 1,458,260); or PA1 4. a release layer of silica (for example, in U.S. Pat. No. 4,357,395). PA1 1. The release layer is easy to apply. PA1 2. The peel strength (A) between the ultra-thin copper foil and the carrier layer is uniform, and has a relatively low value compared to the peel strength (B) of the copper foil after lamination to a substrate. PA1 3. Mechanical polishing and pickling to remove an inorganic material which remains on the surface of an ultra-thin copper foil is not necessary since no inorganic material is used. Thus, the formation of wiring patterns is simplified by reducing the number of processing steps. PA1 4. Although peel strength (A) is low, it is sufficient to prevent separation of the ultra-thin copper foil from the carrier layer during handling. PA1 5. The composite foil has sufficient peel strength (B) after lamination to a substrate and the ultra-thin copper foil does not separate from the substrate during processing into a printed circuit board. PA1 6. The carrier layer can be separated from the ultra-thin copper foil even after laminating at elevated temperatures. PA1 7. It is easy to recycle the carrier layer after it has been separated from since the residual release layer is easy to remove. PA1 1. They are believed to form a chemical bond with copper. PA1 2. They retain the ability to separate a copper carrier from a copper foil even after exposure to the temperatures used in laminating the copper foil to an insulating substrate, preferably at a temperature of not less than 150.degree. C., particularly in the range of about 175-200.degree. C. PA1 3. They form a chemical bond with the ultra-thin copper foil and the carrier layer, and give a peel strength (A) of the carrier layer from the ultra-thin copper foil which is relatively low compared to the peel strength (B) between the ultra-thin copper foil and the insulating substrate. The peel strength (A) is sufficient to prevent their separation during handling and lamination, but low enough to permit the carrier layer to easily be removed after the composite foil has been laminated to the substrate. PA1 4. They provide a very thin release layer which permits uniform electrodeposition of the copper foil on the carrier layer.
Such conventional supported copper foils have, however, been found to present problems.
When the release layer is not uniform over the surface of the carrier layer, bond strength between the carrier layer and the ultra-thin copper foil is uneven. Consequently, when the carrier layer is peeled off after laminating a composite foil, some of the ultra-thin copper foil may remain on the carrier layer or some of the carrier layer may remain on the ultra-thin copper foil. In either case, the required circuit pattern cannot be made. Further, weak bond strength may cause an ultra-thin copper foil to partially or entirely separate from the carrier layer during production and use of the composite foil.
When oxides, sulfides, chromium or inorganic materials, such as chromium or the like, are used as release layers, some of the inorganic material remains on the surface of the ultra-thin copper foil after the carrier layer is peeled off. This inorganic material must be removed before the circuit patterns are made, making it necessary to add extra processing steps.
Finally, when the composite foil is laminated to a substrate, such as an epoxy prepreg at high temperatures, it often becomes difficult to peel off the carrier layer.
Because of these problems, composites of ultra-thin copper foil on a support layer are not generally used in industry at present, despite the proposed methods just discussed.
Accordingly, an object of the present invention is to provide a composite foil which overcomes the problems discussed above and a process for making such composite foils. Another object of the present invention is to provide a copper-clad laminate which is made by using such a composite foil, and a printed wiring board employing such a copper-clad laminate.
The inventors have investigated the metals and/or metal compounds which have conventionally been suggested as release layers for composite foils in the prior art. They have found that when peeling off a support layer from an ultra-thin copper foil after the copper foil has been laminated to a resin substrate by hot pressing, the peel strength is variable and the bond can be too strong. Also, when a release layer is not uniformly formed, or when heat is used during laminating, the metal used in the release layer may be diffused into both the support layer and the ultra-thin copper foil.
The inventors have also investigated organic compounds which are suitable for a release layer of the composite foils which are used for preparing printed wiring boards.
In U.S. Pat. No. 3,281,339, benzothiazole (BTA) was disclosed to be useful as a stripping agent in production of copper sheets or foils by electrodeposition. The principal use of BTA was to make possible a continuous production of a copper sheet using a revolving drum cathode in which the copper sheet electrodeposited on a BTA-coated surface of the revolving drum cathode is continuously separated from the surface. However, the U.S. patent is completely silent on the composite foils used for preparing printed wiring boards, and the properties and materials suitable for the release layers used for the composite foils.
The present invention has been made based on the present inventors' finding that it is possible to employ certain organic compounds as the release layers of the composite foils, even when they are subjected to the elevated temperatures required for lamination of printed circuit boards.